1. Field of the Invention
The present invention relates to a portable telephone system and a portable telephone, and more particularly to a portable telephone system which is composed of a base station connected to a public telephone network, and a portable telephone as a mobile station connectable to the base station by a radio frequency channel, and in which communication between the base station and the portable telephone is limited to a specific area.
2. Description of Related Art
Conventionally, in a portable telephone system, the service area of a plurality of base stations which are connected to a public telephone network is totally predetermined based on all factors such as geography, electromagnetic wave transmission characteristic, a traffic quantity, and communication quality. The plurality of base stations are installed such that the service area can be sufficiently covered. Here, an area which can be covered by one base station is referred to a "cell". The line design is performed in such a manner that it is possible to transmit a call from the portable telephone to the base station or to receive a call from the base station to the portable telephone, if the portable telephone as the mobile station is within the cell.
Here, the base station and the portable telephone as a mobile station have the transmission output of several watts such that a radio frequency channel can be mutually established even when the mobile station is located in a building, a car, and a train in addition to outdoors.
However, in the above-mentioned conventional portable telephone system, there is a problem in public use of the portable telephone, for example, in users anywhere communicate aloud using portable telephones, as the number of portable telephones rapidly increases. Also, this electromagnetic wave transmission output of several watts sometimes causes an erroneous operation of electronic equipment when the portable telephone is used nearby the electronic equipment. For example, there has been recently reported an accident in which a heart pump of a patient stopped when the portable telephone is used in a hospital. For this reason, hospitals prohibit use of portable telephones in the hospitals or bringing of the portable telephone into the hospitals.
In Japanese Laid Open Patent Disclosure (JP-A-Heisei 5-14489), it is described that use of a portable telephone is limited as a reception or transmission only unit during a specific time zone. However, in this convention example, it is impossible to limit the use of the portable telephone in a public place such as a hospital.